U.S. Pat. No. 4,781,552 which is incorporated herein by reference, discloses such an apparatus, comprising two coupled helical gears 9, 10 within a stator, at least one of which gears is devoid of a mechanical bearing, and further comprises at least one inlet opening 40 and one delivery opening 40 for a liquid under pressure, while two side plates 21, 22 enclose the stator on either side of the two gears 9, 10 on which they form a lateral seal.
U.S. Pat. No. 5,028,221 describes such an apparatus of which the driving gear 9 alone is provided with mechanical bearings 123.
The stator comprises a flexible envelope 36 subjected externally to centripetal pressure which enables it to ensure a seal on the tip of the teeth of the helical gears 9, 10 located within the envelope 36. The forces of hydrostatic compensation on the side plates 21, 22 and on the envelope 36 originate, on the one hand, from the pressure of the permanent total pressure zone 34 and, on the other hand, from the pressure prevailing in the hydrostatic balancing compensation sectors 38, 38' and 60, 60', 60", respectively, of the envelope and the side plates. These sectors are supplied via channels 43 and 23. Two covers 54, 55 cover the side plates, while a body 49 surrounds the envelope 36.
Internal hydraulic balancing is ensured by a hydraulic winding comprising rotor conduits in the gears 9 and 10 and stator conduits in the side plates 21 and 22 and the envelope 36, the successive commutations between the rotor and stator conduits being ensured by the passage of their ends one before the other in a commutation circle 20. Balancing between the tooth spaces is ensured by the permanent link between the opposing tooth spaces when the number of teeth is even, and the opposing tooth spaces with an offset of a half step when the number of teeth is odd, via the conduits 23 of the side plates and the channels made in the gears. Of course, such link does not exist between the meshing zone 3 and zones 6, respectively, in which are created hydraulic bearings diametrically opposite the meshing point 3 of the gears 9 and 10. In this way, during rotation of gears 9 and 10, there is a hydraulic winding system which links the opposing pairs of teeth into a relationship such as to obtain the same hydraulic pressure in the tooth spaces for diametrically opposite angular positions and to create hydraulic bearings which develop two reverse forces on the gears with a view to ensuring that they mesh in zone 3 without play.
In U.S. Pat. No. 5,028,221, the rotor circuits in the gears 9, 10 are constituted by groups of conduits 102 diametrically opposite on the commutation circle 20 and parallel (or inclined at the value of the angle of the helix) to the axis of the gears 9, 10 and radial 101, joining the conduits 102 opposite by .pi. to form an H. The stator circuits are constituted by grooves 23, links 30 and conduits 43. Supply of high pressure to the permanent total pressure zone 34 is effected by a preferential valve system also allowing decompression of this zone 34 when desired. Seal anti-extrusion devices are incorporated in the hydrostatic compensation sectors 38 and 60 respectively defined by seals 37 and 45, zone 34 being defined outside these seals and closed on the axis of the gears 9 and 10 by the seals 58.
The embodiment according to U.S. Pat. No. 5,028,221 risks deformation of the side plates 21 and 22 made of plastic material in the zones where they present grooves 100 when low pressure prevails therein, while the outside of the side plates is at permanent total high pressure coming from zone 34. Similarly, this embodiment has provided seal anti-extrusion devices also serving as reinforcement for the plastic pieces at the level of the outlet openings 40. These devices are not entirely satisfactory, in that they lead to making the outlet openings 40 solely in the side plates and covers and do not enable them to be made in the envelope 36 and the body 49, which envelope might be deformed by the high pressure.
Finally, for large cubic capacities, the increase in the diameter of the gears leads to a proportionate increase in the number of sectors, which renders production more expensive due to the larger number of sectors and more difficult to balance due to the sectors at intermediate pressures.